I was in a wolver den when this happened. (Will add link soon.) The wolver TP'd outside of the level and was floating. This happened because he was trying to TP next to me but I was already surrounded by wolvers on the side he tried to TP to, so instead the game placed this wolver behind the ones he was trying to TP to, though there was only empty space behind them. I think, due to lack of space, he got shoved into space and stayed there until the TP'd again.
For noob's reference: TP means Teleport, or burrow, which is what they do in T3.
Heh, and you yourself can TP warp past a gate on first floor gitm for the same reason. When that dreadnaught spawns on the last wave of the last button before the elevator, if you're flush in the middle of that gate and hugging it, it'll TP you to the side with the elevator due to pathfind fail-panic because you're surrounded on all sides by monsters.